5Using PL/SQL Collections and Records

Knowledge is that area of ignorance that we arrange and classify. —Ambrose Bierce

Many programming techniques use collection types such as arrays, bags, lists, nested tables, sets, and trees. You can model these types in database applications using the PL/SQL datatypes TABLE and VARRAY, which allow you to declare nested tables, associative arrays, and variable-size arrays. This chapter shows how to reference and manipulate collections of data as local variables. You also learn how the RECORD datatype lets you manipulate related values of different types as a logical unit.

What Is a Collection?

A collection is an ordered group of elements, all of the same type. It is a general concept that encompasses lists, arrays, and other datatypes used in classic programming algorithms. Each element is addressed by a unique subscript.

PL/SQL offers these collection types:

Associative arrays, also known as index-by tables, let you look up elements using arbitrary numbers and strings for subscript values. (They are similar to hash tables in other programming languages.)

Nested tables hold an arbitrary number of elements. They use sequential numbers as subscripts. You can define equivalent SQL types, allowing nested tables to be stored in database tables and manipulated through SQL.

Varrays (short for variable-size arrays) hold a fixed number of elements (although you can change the number of elements at runtime). They use sequential numbers as subscripts. You can define equivalent SQL types, allowing varrays to be stored in database tables. They can be stored and retrieved through SQL, but with less flexibility than nested tables.

Although collections have only one dimension, you can model multi-dimensional arrays by creating collections whose elements are also collections.

To use collections in an application, you define one or more PL/SQL types, then define variables of those types. You can define collection types in a procedure, function, or package. You can pass collection variables as parameters to stored subprograms.

To look up data that is more complex than single values, you can store PL/SQL records or SQL object types in collections. Nested tables and varrays can also be attributes of object types.

Understanding Nested Tables

PL/SQL nested tables represent sets of values. You can think of them as one-dimensional arrays with no upper bound. You can model multi-dimensional arrays by creating nested tables whose elements are also nested tables.

Within the database, nested tables are column types that hold sets of values. Oracle stores the rows of a nested table in no particular order. When you retrieve a nested table from the database into a PL/SQL variable, the rows are given consecutive subscripts starting at 1. That gives you array-like access to individual rows.

Nested tables differ from arrays in two important ways:

Nested tables are unbounded, while arrays have a fixed upper bound (see Figure 5-1). The size of a nested table can increase dynamically.

Nested tables might not have consecutive subscripts, while arrays are always dense (have consecutive subscripts). Initially, nested tables are dense, but they can become sparse (have nonconsecutive subscripts). You can delete elements from a nested table using the built-in procedure DELETE. The built-in function NEXT lets you iterate over all the subscripts of a nested table, even if the sequence has gaps.

Understanding Varrays

Items of type VARRAY are called varrays. They let you reference individual elements for array operations, or manipulate the collection as a whole. To reference an element, you use standard subscripting syntax (see Figure 5-2). For example, Grade(3) references the third element in varray Grades.

A varray has a maximum size, which you specify in its type definition. Its index has a fixed lower bound of 1 and an extensible upper bound. For example, the current upper bound for varray Grades is 7, but you can increase its upper bound to maximum of 10. A varray can contain a varying number of elements, from zero (when empty) to the maximum specified in its type definition.

Understanding Associative Arrays (Index-By Tables)

Associative arrays are sets of key-value pairs, where each key is unique and is used to locate a corresponding value in the array. The key can be an integer or a string.

Assigning a value using a key for the first time adds that key to the associative array. Subsequent assignments using the same key update the same entry. It is important to choose a key that is unique. For example, key values might come from the primary key of a database table, from a numeric hash function, or from concatenating strings to form a unique string value.

For example, here is the declaration of an associative array type, and two arrays of that type, using keys that are strings:

Associative arrays help you represent data sets of arbitrary size, with fast lookup for an individual element without knowing its position within the array and without having to loop through all the array elements. It is like a simple version of a SQL table where you can retrieve values based on the primary key. For simple temporary storage of lookup data, associative arrays let you avoid using the disk space and network operations required for SQL tables.

Because associative arrays are intended for temporary data rather than storing persistent data, you cannot use them with SQL statements such as INSERT and SELECT INTO. You can make them persistent for the life of a database session by declaring the type in a package and assigning the values in a package body.

If settings for national language or globalization change during a session that uses associative arrays with VARCHAR2 key values, the program might encounter a runtime error. For example, changing the NLS_COMP or NLS_SORT initialization parameters within a session might cause methods such as NEXT and PRIOR to raise exceptions. If you need to change these settings during the session, make sure to set them back to their original values before performing further operations with these kinds of associative arrays.

When you declare an associative array using a string as the key, the declaration must use a VARCHAR2, STRING, or LONG type. You can use a different type, such as NCHAR or NVARCHAR2, as the key value to reference an associative array. You can even use a type such as DATE, as long as it can be converted to VARCHAR2 by the TO_CHAR function.

However, you must be careful when using other types that the values used as keys are consistent and unique. For example, the string value of SYSDATE might change if the NLS_DATE_FORMAT initialization parameter changes, so that array_element(SYSDATE) does not produce the same result as before. Two different NVARCHAR2 values might turn into the same VARCHAR2 value (containing question marks instead of certain national characters). In that case, array_element(national_string1) and array_element(national_string2) might refer to the same element. Two different CHAR or VARCHAR2 values that differ in terms of case, accented characters, or punctuation characters might also be considered the same if the value of the NLS_SORT initialization parameter ends in _CI (case-insensitive comparisons) or _AI (accent- and case-insensitive comparisons).

When you pass an associative array as a parameter to a remote database using a database link, the two databases can have different globalization settings. When the remote database performs operations such as FIRST and NEXT, it uses its own character order even if that is different from the order where the collection originated. If character set differences mean that two keys that were unique are not unique on the remote database, the program receives a VALUE_ERROR exception.

Choosing Which PL/SQL Collection Types to Use

If you already have code or business logic that uses some other language, you can usually translate that language's array and set types directly to PL/SQL collection types.

Arrays in other languages become varrays in PL/SQL.

Sets and bags in other languages become nested tables in PL/SQL.

Hash tables and other kinds of unordered lookup tables in other languages become associative arrays in PL/SQL.

When you are writing original code or designing the business logic from the start, you should consider the strengths of each collection type to decide which is appropriate for each situation.

Choosing Between Nested Tables and Associative Arrays

Both nested tables and associative arrays (formerly known as index-by tables) use similar subscript notation, but they have different characteristics when it comes to persistence and ease of parameter passing.

Nested tables can be stored in a database column, but associative arrays cannot. Nested tables can simplify SQL operations where you would normally join a single-column table with a larger table.

Associative arrays are appropriate for relatively small lookup tables where the collection can be constructed in memory each time a procedure is called or a package is initialized. They are good for collecting information whose volume is unknown beforehand, because there is no fixed limit on their size. Their index values are more flexible, because associative array subscripts can be negative, can be nonsequential, and can use string values instead of numbers.

PL/SQL automatically converts between host arrays and associative arrays that use numeric key values. The most efficient way to pass collections to and from the database server is to set up data values in associative arrays, then use those associative arrays with bulk constructs (the FORALL statement or BULK COLLECT clause).

Choosing Between Nested Tables and Varrays

Varrays are a good choice when:

The number of elements is known in advance.

The elements are usually all accessed in sequence.

When stored in the database, varrays keep their ordering and subscripts.

Each varray is stored as a single object, either inside the table of which it is a column (if the varray is less than 4KB) or outside the table but still in the same tablespace (if the varray is greater than 4KB). You must update or retrieve all elements of the varray at the same time, which is most appropriate when performing some operation on all the elements at once. But you might find it impractical to store and retrieve large numbers of elements this way.

Nested tables are a good choice when:

The index values are not consecutive.

There is no predefined upper bound for index values.

You need to delete or update some elements, but not all the elements at once.

You would usually create a separate lookup table, with multiple entries for each row of the main table, and access it through join queries.

Nested tables can be sparse: you can delete arbitrary elements, rather than just removing an item from the end.

Nested table data is stored in a separate store table, a system-generated database table associated with the nested table. The database joins the tables for you when you access the nested table. This makes nested tables suitable for queries and updates that only affect some elements of the collection.

You cannot rely on the order and subscripts of a nested table remaining stable as the nested table is stored in and retrieved from the database, because the order and subscripts are not preserved in the database.

Defining Collection Types

To create collections, you define a collection type, then declare variables of that type. You can define TABLE and VARRAY types in the declarative part of any PL/SQL block, subprogram, or package.

Collections follow the same scoping and instantiation rules as other types and variables. Collections are instantiated when you enter a block or subprogram, and cease to exist when you exit. In a package, collections are instantiated when you first reference the package and cease to exist when you end the database session.

Nested Tables

To define a PL/SQL type for nested tables, use the syntax:

TYPE type_name IS TABLE OF element_type [NOT NULL];

type_name is a type specifier used later to declare collections. For nested tables declared within PL/SQL, element_type is any PL/SQL datatype except:

REF CURSOR

Nested tables declared in SQL using the CREATE TYPE statement have additional restrictions on the element type. They cannot use the following element types:

The key_type can be numeric, either PLS_INTEGER or BINARY_INTEGER. It can also be VARCHAR2 or one of its subtypes VARCHAR, STRING, or LONG. You must specify the length of a VARCHAR2-based key, except for LONG which is equivalent to declaring a key type of VARCHAR2(32760). The types RAW, LONG RAW, ROWID, CHAR, and CHARACTER are not allowed as keys for an associative array.

An initialization clause is not allowed. There is no constructor notation for associative arrays.

When you reference an element of an associative array that uses a VARCHAR2-based key, you can use other types, such as DATE or TIMESTAMP, as long as they can be converted to VARCHAR2 with the TO_CHAR function.

Associative arrays can store data using a primary key value as the index, where the key values are not sequential. The example below creates a single element in an associative array, with a subscript of 100 rather than 1:

The identifier courses represents an entire nested table. Each element of courses stores the name of a college course such as 'Math 1020'.

Example 5-2 Creating a Table with a Varray Column

The script below creates a database column that stores varrays. Each varray element contains a VARCHAR2.

-- Each project has a 16-character code name.
-- We will store up to 50 projects at a time in a database column.
CREATE TYPE ProjectList AS VARRAY(50) OF VARCHAR2(16);
/
CREATE TABLE department ( -- create database table
dept_id NUMBER(2),
name VARCHAR2(15),
budget NUMBER(11,2),
-- Each department can have up to 50 projects.
projects ProjectList);
DROP TABLE department;
DROP TYPE ProjectList;

Declaring PL/SQL Collection Variables

After defining a collection type, you declare variables of that type. You use the new type name in the declaration, the same as with predefined types such as NUMBER.

You can use %TYPE to specify the datatype of a previously declared collection, so that changing the definition of the collection automatically updates other variables that depend on the number of elements or the element type:

DECLARE
TYPE Few_Colors IS VARRAY(10) OF VARCHAR2(20);
TYPE Many_Colors IS VARRAY(100) OF VARCHAR2(64);
some_colors Few_Colors;
-- If we change the type of SOME_COLORS from FEW_COLORS to MANY_COLORS,
-- RAINBOW and CRAYONS will use the same type when this block is recompiled.
rainbow some_colors%TYPE;
crayons some_colors%TYPE;
BEGIN
NULL;
END;
/

Example 5-5 Declaring a Procedure Parameter as a Nested Table

You can declare collections as the formal parameters of functions and procedures. That way, you can pass collections to stored subprograms and from one subprogram to another. The following example declares a nested table as a parameter of a packaged procedure:

To specify the element type, you can use %TYPE, which provides the datatype of a variable or database column. Also, you can use %ROWTYPE, which provides the rowtype of a cursor or database table. Two examples follow:

DECLARE
-- Nested table type that can hold an arbitrary number of
-- employee IDs. The element type is based on a column from
-- the EMPLOYEES table. We do not need to know whether the
-- ID is a number or a string.
TYPE EmpList IS TABLE OF employees.employee_id%TYPE;
-- Array type that can hold information about 10 employees.
-- The element type is a record that contains all the same
-- fields as the EMPLOYEES table.
TYPE Top_Salespeople IS VARRAY(10) OF employees%ROWTYPE;
-- Declare a cursor to select a subset of columns.
CURSOR c1 IS SELECT first_name, last_name FROM employees;
-- Array type that can hold a list of names. The element type
-- is a record that contains the same fields as the cursor
-- (that is, first_name and last_name).
TYPE NameList IS VARRAY(20) OF c1%ROWTYPE;
BEGIN
NULL;
END;
/

Initializing and Referencing Collections

Until you initialize it, a nested table or varray is atomically null: the collection itself is null, not its elements. To initialize a nested table or varray, you use a constructor, a system-defined function with the same name as the collection type. This function "constructs" collections from the elements passed to it.

You must explicitly call a constructor for each varray and nested table variable. (Associative arrays, the third kind of collection, do not use constructors.) Constructor calls are allowed wherever function calls are allowed.

Example 5-9 Constructor for a Nested Table

The following example initializes a nested table using a constructor, which looks like a function with the same name as the collection type:

Because a nested table does not have a declared maximum size, you can put as many elements in the constructor as necessary.

Example 5-10 Constructor for a Varray

This example initializes a varray using a constructor, which looks like a function with the same name as the collection type:

DECLARE
-- In the varray, we put an upper limit on the number of elements.
TYPE Colors IS VARRAY(10) OF VARCHAR2(16);
rainbow Colors;
BEGIN
-- Since COLORS is declared as VARRAY(10), we can put up to 10
-- elements in the constructor.
rainbow := Colors('Red','Orange','Yellow','Green','Blue','Indigo','Violet');
END;
/

Example 5-11 Collection Constructor Including Null Elements

Unless you impose the NOTNULL constraint in the type declaration, you can pass null elements to a constructor:

DECLARE
TYPE Colors IS TABLE OF VARCHAR2(20);
my_colors Colors;
TYPE ColorsNoNulls IS TABLE OF VARCHAR2(20) NOT NULL;
BEGIN
my_colors := Colors('Sienna',NULL,'Teal','Umber',NULL);
-- If MY_COLORS was of type ColorsNoNulls, we could not include
-- null values in the constructor.
END;
/

Example 5-12 Combining Collection Declaration and Constructor

You can initialize a collection in its declaration, which is a good programming practice:

Referencing Collection Elements

Every reference to an element includes a collection name and a subscript enclosed in parentheses. The subscript determines which element is processed. To reference an element, you specify its subscript using the syntax

collection_name(subscript)

where subscript is an expression that yields an integer in most cases, or a VARCHAR2 for associative arrays declared with strings as keys.

The allowed subscript ranges are:

For nested tables, 1 .. 2**31.

For varrays, 1 .. size_limit, where you specify the limit in the declaration.

For associative arrays with a numeric key, -2**31 .. 2**31.

For associative arrays with a string key, the length of the key and number of possible values depends on the VARCHAR2 length limit in the type declaration, and the database character set.

Example 5-16 Referencing a Nested Table Element By Subscript

This example shows how to reference an element in the nested table NAMES:

Assigning Collections

One collection can be assigned to another by an INSERT, UPDATE, FETCH, or SELECT statement, an assignment statement, or a subprogram call.

You can assign the value of an expression to a specific element in a collection using the syntax:

collection_name(subscript) := expression;

where expression yields a value of the type specified for elements in the collection type definition.

You can use operators such as SET, MULTISET UNION, MULTISET INTERSECT, and MULTISET EXCEPT to transform nested tables as part of an assignment statement.

Example 5-18 Datatype Compatibility for Collection Assignment

This example shows that collections must have the same datatype for an assignment to work. Having the same element type is not enough.

DECLARE
TYPE last_name_typ IS VARRAY(3) OF VARCHAR2(64);
TYPE surname_typ IS VARRAY(3) OF VARCHAR2(64);
-- These first two variables have the same datatype.
group1 last_name_typ := last_name_typ('Jones','Wong','Marceau');
group2 last_name_typ := last_name_typ('Klein','Patsos','Singh');
-- This third variable has a similar declaration, but is not the same type.
group3 surname_typ := surname_typ('Trevisi','Macleod','Marquez');
BEGIN
-- Allowed because they have the same datatype
group1 := group2;
-- Not allowed because they have different datatypes
-- group3 := group2;
END;
/

Example 5-19 Assigning a Null Value to a Nested Table

If you assign an atomically null nested table or varray to a second nested table or varray, the second collection must be reinitialized:

DECLARE
TYPE Colors IS TABLE OF VARCHAR2(64);
-- This nested table has some values.
crayons Colors := Colors('Silver','Gold');
-- This nested table is not initialized ("atomically null").
empty_set Colors;
BEGIN
-- At first, the initialized variable is not null.
if crayons IS NOT NULL THEN
dbms_output.put_line('OK, at first crayons is not null.');
END IF;
-- Then we assign a null nested table to it.
crayons := empty_set;
-- Now it is null.
if crayons IS NULL THEN
dbms_output.put_line('OK, now crayons has become null.');
END IF;
-- We must use another constructor to give it some values.
crayons := Colors('Yellow','Green','Blue');
END;
/

In the same way, assigning the value NULL to a collection makes it atomically null.

Example 5-20 Possible Exceptions for Collection Assignments

Assigning a value to a collection element can cause various exceptions:

If the subscript is null or is not convertible to the right datatype, PL/SQL raises the predefined exception VALUE_ERROR. Usually, the subscript must be an integer. Associative arrays can also be declared to have VARCHAR2 subscripts.

If the subscript refers to an uninitialized element, PL/SQL raises SUBSCRIPT_BEYOND_COUNT.

If the collection is atomically null, PL/SQL raises COLLECTION_IS_NULL.

DECLARE
TYPE WordList IS TABLE OF VARCHAR2(5);
words WordList;
BEGIN
/* Assume execution continues despite the raised exceptions. */
-- Raises COLLECTION_IS_NULL. We haven't used a constructor yet.
-- This exception applies to varrays and nested tables, but not to
-- associative arrays which don't need a constructor.
words(1) := 10;
-- After using a constructor, we can assign values to the elements.
words := WordList(10,20,30);
-- Any expression that returns a VARCHAR2(5) is OK.
words(1) := 'yes';
words(2) := words(1) || 'no';
-- Raises VALUE_ERROR because the assigned value is too long.
words(3) := 'longer than 5 characters';
-- Raises VALUE_ERROR because the subscript of a nested table must
-- be an integer.
words('B') := 'dunno';
-- Raises SUBSCRIPT_BEYOND_COUNT because we only made 3 elements
-- in the constructor. To add new ones, we must call the EXTEND
-- method first.
words(4) := 'maybe';
END;
/

Example 5-21 Assigning Nested Tables with Set Operators

This example shows some of the ANSI-standard operators that you can apply to nested tables:

Comparing Collections

You can check whether a collection is null, and whether two collections are the same. Comparisons such as greater than, less than, and so on are not allowed.

This restriction also applies to implicit comparisons. For example, collections cannot appear in a DISTINCT, GROUPBY, or ORDERBY list.

If you want to do such comparison operations, you must define your own notion of what it means for collections to be greater than, less than, and so on, and write one or more functions to examine the collections and their elements and return a true or false value.

You can apply set operators (CARDINALITY, MEMBER OF, IS A SET, IS EMPTY) to check certain conditions within a nested table or between two nested tables.

Example 5-22 Checking if a Collection Is Null

Nested tables and varrays can be atomically null, so they can be tested for nullity:

DECLARE
TYPE Staff IS TABLE OF Employee;
members Staff;
BEGIN
-- Condition yields TRUE because we haven't used a constructor.
IF members IS NULL THEN ...
END;

Example 5-23 Comparing Two Collections

Collections can be compared for equality or inequality. They cannot be ordered, because there is no "greater than" or "less than" comparison.

DECLARE
TYPE Colors IS TABLE OF VARCHAR2(64);
primaries Colors := Colors('Blue','Green','Red');
rgb Colors := Colors('Red','Green','Blue');
traffic_light Colors := Colors('Red','Green','Amber');
BEGIN
-- We can use = or !=, but not < or >.
-- Notice that these 2 are equal even though the members are in different order.
IF primaries = rgb THEN
dbms_output.put_line('OK, PRIMARIES and RGB have the same members.');
END IF;
IF rgb != traffic_light THEN
dbms_output.put_line('OK, RGB and TRAFFIC_LIGHT have different members.');
END IF;
END;
/

Example 5-24 Comparing Nested Tables with Set Operators

You can test certain properties of a nested table, or compare two nested tables, using ANSI-standard set operations:

Each item in column COURSES is a nested table that will store the courses offered by a given department. The NESTEDTABLE clause is required whenever a database table has a nested table column. The clause identifies the nested table and names a system-generated store table, in which Oracle stores the nested table data.

Example 5-26 Inserting a Nested Table into a Database Table

Now, you can populate the database table. The table constructor provides values that all go into the single column COURSES:

You can retrieve all the courses offered by the English department into a PL/SQL nested table:

CREATE TYPE ColorList AS TABLE OF VARCHAR2(64);
/
CREATE TABLE flowers (name VARCHAR2(20), colors ColorList) NESTED TABLE colors STORE AS colors_tab;
BEGIN
INSERT INTO flowers VALUES('Rose', ColorList('Red','Yellow','White'));
INSERT INTO flowers VALUES('Tulip', ColorList('Red','White','Yellow', 'Blue'));
INSERT INTO flowers VALUES('Iris', ColorList('White','Purple'));
COMMIT;
END;
/
DECLARE
-- This type declaration is not needed, because PL/SQL can see the SQL type.
-- TYPE ColorList IS TABLE OF VARCHAR2(64);
-- Declare a variable that can hold a set of colors.
my_colors ColorList;
-- Declare a record that can hold a row from the table.
-- One of the record fields is a set of colors.
my_flower flowers%ROWTYPE;
new_colors ColorList;
BEGIN
-- Look up a name and query just the associated colors.
SELECT colors INTO my_colors FROM flowers WHERE name = 'Rose';
FOR i IN my_colors.FIRST .. my_colors.LAST
LOOP
dbms_output.put_line('Rose color = ' || my_colors(i));
END LOOP;
-- Look up a name and query the entire row.
SELECT * INTO my_flower FROM flowers WHERE name = 'Iris';
-- Now COLORS is a field in a record, so we access it with dot notation.
FOR i IN my_flower.colors.FIRST .. my_flower.colors.LAST
LOOP
-- Because we have all the table columns in the record, we can refer to NAME also.
dbms_output.put_line(my_flower.name || ' color = ' || my_flower.colors(i));
END LOOP;
-- We can replace a set of colors by making a new collection and using it
-- in an UPDATE statement.
new_colors := ColorList('Red','Yellow','White','Pink');
UPDATE flowers SET colors = new_colors WHERE name = 'Rose';
-- Or we can modify the original collection and use it in the UPDATE.
-- We'll add a new final element and fill in a value.
my_flower.colors.EXTEND(1);
my_flower.colors(my_flower.colors.COUNT) := 'Yellow';
UPDATE flowers SET colors = my_flower.colors WHERE name = my_flower.name;
-- We can even treat the nested table column like a real table and
-- insert, update, or delete elements.
-- The TABLE operator makes the statement apply to the nested table produced by the subquery.
INSERT INTO TABLE(SELECT colors FROM flowers WHERE name = 'Rose') VALUES('Black');
DELETE FROM TABLE(SELECT colors FROM flowers WHERE name = 'Rose') WHERE column_value = 'Yellow';
UPDATE TABLE(SELECT colors FROM flowers WHERE name = 'Iris')
SET column_value = 'Indigo' WHERE column_value = 'Purple';
COMMIT;
END;
/
DROP TABLE flowers;
DROP TYPE ColorList;

Within PL/SQL, you can manipulate the nested table by looping through its elements, using methods such as TRIM or EXTEND, and updating some or all of the elements. Afterwards, you can store the updated table in the database again.

Using PL/SQL Varrays with INSERT, UPDATE, and SELECT Statements

This example shows how you can transfer varrays between PL/SQL variables and SQL tables. You can insert table rows containing varrays, update a row to replace its varray, and select varrays into PL/SQL variables. You cannot update or delete individual varray elements directly with SQL; you have to select the varray from the table, change it in PL/SQL, then update the table to include the new varray.

Manipulating Individual Collection Elements with SQL

By default, SQL operations store and retrieve whole collections rather than individual elements. To manipulate the individual elements of a collection with SQL, use the TABLE operator. The TABLE operator uses a subquery to extract the varray or nested table, so that the INSERT, UPDATE, or DELETE statement applies to the nested table rather than the top-level table.

To perform DML operations on a PL/SQL nested table, use the operators TABLE and CAST. This way, you can do set operations on nested tables using SQL notation, without actually storing the nested tables in the database.

The operands of CAST are PL/SQL collection variable and a SQL collection type (created by the CREATE TYPE statement). CAST converts the PL/SQL collection to the SQL type.

Using Multilevel Collections

In addition to collections of scalar or object types, you can also create collections whose elements are collections. For example, you can create a nested table of varrays, a varray of varrays, a varray of nested tables, and so on.

When creating a nested table of nested tables as a column in SQL, check the syntax of the CREATE TABLE statement to see how to define the storage table.

Here are some examples showing the syntax and possibilities for multilevel collections.

Using Collection Methods

These collection methods make collections easier to use, and make your applications easier to maintain:

EXISTS

COUNT

LIMIT

FIRST and LAST

PRIOR and NEXT

EXTEND

TRIM

DELETE

A collection method is a built-in function or procedure that operates on collections and is called using dot notation.

Collection methods cannot be called from SQL statements.

EXTEND and TRIM cannot be used with associative arrays.

EXISTS, COUNT, LIMIT, FIRST, LAST, PRIOR, and NEXT are functions; EXTEND, TRIM, and DELETE are procedures.

EXISTS, PRIOR, NEXT, TRIM, EXTEND, and DELETE take parameters corresponding to collection subscripts, which are usually integers but can also be strings for associative arrays.

Only EXISTS can be applied to atomically null collections. If you apply another method to such collections, PL/SQL raises COLLECTION_IS_NULL.

Checking If a Collection Element Exists (EXISTS Method)

EXISTS(n) returns TRUE if the nth element in a collection exists. Otherwise, EXISTS(n) returns FALSE. By combining EXISTS with DELETE, you can work with sparse nested tables. You can also use EXISTS to avoid referencing a nonexistent element, which raises an exception. When passed an out-of-range subscript, EXISTS returns FALSE instead of raising SUBSCRIPT_OUTSIDE_LIMIT.

DECLARE
TYPE NumList IS TABLE OF INTEGER;
n NumList := NumList(1,3,5,7);
BEGIN
n.DELETE(2); -- Delete the second element
IF n.EXISTS(1) THEN
dbms_output.put_line('OK, element #1 exists.');
END IF;
IF n.EXISTS(2) = FALSE THEN
dbms_output.put_line('OK, element #2 has been deleted.');
END IF;
IF n.EXISTS(99) = FALSE THEN
dbms_output.put_line('OK, element #99 does not exist at all.');
END IF;
END;
/

Counting the Elements in a Collection (COUNT Method)

COUNT returns the number of elements that a collection currently contains:

COUNT is useful because the current size of a collection is not always known. For example, you can fetch a column of Oracle data into a nested table, where the number of elements depends on the size of the result set.

For varrays, COUNT always equals LAST. You can increase or decrease the size of a varray using the EXTEND and TRIM methods, so the value of COUNT can change, up to the value of the LIMIT method.

Checking the Maximum Size of a Collection (LIMIT Method)

For nested tables and associative arrays, which have no maximum size, LIMIT returns NULL. For varrays, LIMIT returns the maximum number of elements that a varray can contain/ You specify this limit in the type definition, and can change it later with the TRIM and EXTEND methods. For instance, if the maximum size of varray PROJECTS is 25 elements, the following IF condition is true:

Finding the First or Last Collection Element (FIRST and LAST Methods)

FIRST and LAST return the first and last (smallest and largest) index numbers in a collection that uses integer subscripts.

For an associative array with VARCHAR2 key values, the lowest and highest key values are returned. By default, the order is based on the binary values of the characters in the string. If the NLS_COMP initialization parameter is set to ANSI, the order is based on the locale-specific sort order specified by the NLS_SORT initialization parameter.

If the collection is empty, FIRST and LAST return NULL.

If the collection contains only one element, FIRST and LAST return the same index value.

The following example shows how to use FIRST and LAST to iterate through the elements in a collection that has consecutive subscripts:

DECLARE
TYPE NumList IS TABLE OF NUMBER;
n NumList := NumList(1,3,5,7);
counter INTEGER;
BEGIN
dbms_output.put_line('N''s first subscript is ' || n.FIRST);
dbms_output.put_line('N''s last subscript is ' || n.LAST);
-- When the subscripts are consecutive starting at 1, it's simple to loop through them.
FOR i IN n.FIRST .. n.LAST
LOOP
dbms_output.put_line('Element #' || i || ' = ' || n(i));
END LOOP;
n.DELETE(2); -- Delete second element.
-- When the subscripts have gaps or the collection might be uninitialized,
-- the loop logic is more extensive. We start at the first element, and
-- keep looking for the next element until there are no more.
IF n IS NOT NULL THEN
counter := n.FIRST;
WHILE counter IS NOT NULL
LOOP
dbms_output.put_line('Element #' || counter || ' = ' || n(counter));
counter := n.NEXT(counter);
END LOOP;
ELSE
dbms_output.put_line('N is null, nothing to do.');
END IF;
END;
/

For varrays, FIRST always returns 1 and LAST always equals COUNT.

For nested tables, normally FIRST returns 1 and LAST equals COUNT. But if you delete elements from the beginning of a nested table, FIRST returns a number larger than 1. If you delete elements from the middle of a nested table, LAST becomes larger than COUNT.

When scanning elements, FIRST and LAST ignore deleted elements.

Looping Through Collection Elements (PRIOR and NEXT Methods)

PRIOR(n) returns the index number that precedes index n in a collection. NEXT(n) returns the index number that succeeds index n. If n has no predecessor, PRIOR(n) returns NULL. If n has no successor, NEXT(n) returns NULL.

For associative arrays with VARCHAR2 keys, these methods return the appropriate key value; ordering is based on the binary values of the characters in the string, unless the NLS_COMP initialization parameter is set to ANSI, in which case the ordering is based on the locale-specific sort order specified by the NLS_SORT initialization parameter.

These methods are more reliable than looping through a fixed set of subscript values, because elements might be inserted or deleted from the collection during the loop. This is especially true for associative arrays, where the subscripts might not be in consecutive order and so the sequence of subscripts might be (1,2,4,8,16) or ('A','E','I','O','U').

DECLARE
TYPE NumList IS TABLE OF NUMBER;
n NumList := NumList(1966,1971,1984,1989,1999);
BEGIN
dbms_output.put_line('The element after #2 is #' || n.NEXT(2));
dbms_output.put_line('The element before #2 is #' || n.PRIOR(2));
n.DELETE(3); -- Delete an element to show how NEXT can handle gaps.
dbms_output.put_line('Now the element after #2 is #' || n.NEXT(2));
IF n.PRIOR(n.FIRST) IS NULL THEN
dbms_output.put_line('Can''t get PRIOR of the first element or NEXT of the last.');
END IF;
END;
/

You can use PRIOR or NEXT to traverse collections indexed by any series of subscripts. The following example uses NEXT to traverse a nested table from which some elements have been deleted:

If you impose the NOTNULL constraint on a TABLE or VARRAY type, you cannot apply the first two forms of EXTEND to collections of that type.

EXTEND operates on the internal size of a collection, which includes any deleted elements. If EXTEND encounters deleted elements, it includes them in its tally. PL/SQL keeps placeholders for deleted elements so that you can re-create them by assigning new values.

When it includes deleted elements, the internal size of a nested table differs from the values returned by COUNT and LAST. For instance, if you initialize a nested table with five elements, then delete elements 2 and 5, the internal size is 5, COUNT returns 3, and LAST returns 4. All deleted elements, regardless of position, are treated alike.

Decreasing the Size of a Collection (TRIM Method)

This procedure has two forms:

TRIM removes one element from the end of a collection.

TRIM(n) removes n elements from the end of a collection.

For example, this statement removes the last three elements from nested table courses:

In general, do not depend on the interaction between TRIM and DELETE. It is better to treat nested tables like fixed-size arrays and use only DELETE, or to treat them like stacks and use only TRIM and EXTEND.

Because PL/SQL does not keep placeholders for trimmed elements, you cannot replace a trimmed element simply by assigning it a new value.

Deleting Collection Elements (DELETE Method)

This procedure has various forms:

DELETE removes all elements from a collection.

DELETE(n) removes the nth element from an associative array with a numeric key or a nested table. If the associative array has a string key, the element corresponding to the key value is deleted. If n is null, DELETE(n) does nothing.

DELETE(m,n) removes all elements in the range m..n from an associative array or nested table. If m is larger than n or if m or n is null, DELETE(m,n) does nothing.

Varrays always have consecutive subscripts, so you cannot delete individual elements except from the end (by using the TRIM method).

If an element to be deleted does not exist, DELETE simply skips it; no exception is raised. PL/SQL keeps placeholders for deleted elements, so you can replace a deleted element by assigning it a new value.

DELETE lets you maintain sparse nested tables. You can store sparse nested tables in the database, just like any other nested tables.

The amount of memory allocated to a nested table can increase or decrease dynamically. As you delete elements, memory is freed page by page. If you delete the entire table, all the memory is freed.

Applying Methods to Collection Parameters

Within a subprogram, a collection parameter assumes the properties of the argument bound to it. You can apply the built-in collection methods (FIRST, LAST, COUNT, and so on) to such parameters. You can create general-purpose subprograms that take collection parameters and iterate through their elements, add or delete elements, and so on.

Note: For varray parameters, the value of LIMIT is always derived from the parameter type definition, regardless of the parameter mode.

Avoiding Collection Exceptions

In most cases, if you reference a nonexistent collection element, PL/SQL raises a predefined exception. Consider the following example:

In the first case, the nested table is atomically null. In the second case, the subscript is null. In the third case, the subscript is outside the allowed range. In the fourth case, the subscript exceeds the number of elements in the table. In the fifth case, the subscript designates a deleted element.

The following list shows when a given exception is raised:

Collection Exception

Raised when...

COLLECTION_IS_NULL

you try to operate on an atomically null collection.

NO_DATA_FOUND

a subscript designates an element that was deleted, or a nonexistent element of an associative array.

SUBSCRIPT_BEYOND_COUNT

a subscript exceeds the number of elements in a collection.

SUBSCRIPT_OUTSIDE_LIMIT

a subscript is outside the allowed range.

VALUE_ERROR

a subscript is null or not convertible to the key type. This exception might occur if the key is defined as a PLS_INTEGER range, and the subscript is outside this range.

In some cases, you can pass invalid subscripts to a method without raising an exception. For instance, when you pass a null subscript to procedure DELETE, it does nothing. You can replace deleted elements by assigning values to them, without raising NO_DATA_FOUND:

The second procedure call fails, because the packaged and local VARRAY types are incompatible despite their identical definitions.

What Is a PL/SQL Record?

A record is a group of related data items stored in fields, each with its own name and datatype. You can think of a record as a variable that can hold a table row, or some columns from a table row. The fields correspond to table columns.

The %ROWTYPE attribute lets you declare a record that represents a row in a database table, without listing all the columns. Your code keeps working even after columns are added to the table. If you want to represent a subset of columns in a table, or columns from different tables, you can define a view or declare a cursor to select the right columns and do any necessary joins, and then apply %ROWTYPE to the view or cursor.

Defining and Declaring Records

To create records, you define a RECORD type, then declare records of that type. You can also create or find a table, view, or PL/SQL cursor with the values you want, and use the %ROWTYPE attribute to create a matching record.

You can define RECORD types in the declarative part of any PL/SQL block, subprogram, or package. When you define your own RECORD type, you can specify a NOT NULL constraint on fields, or give them default values.

DECLARE
-- Declare a record type with 3 fields.
TYPE rec1_t IS RECORD (field1 VARCHAR2(16), field2 NUMBER, field3 DATE);
-- For any fields declared NOT NULL, we must supply a default value.
TYPE rec2_t IS RECORD (id INTEGER NOT NULL := -1, name VARCHAR2(64) NOT NULL := '[anonymous]');
-- Declare record variables of the types declared above.
rec1 rec1_t;
rec2 rec2_t;
-- Declare a record variable that can hold a row from the EMPLOYEES table.
-- The fields of the record automatically match the names and types of the columns.
-- Don't need a TYPE declaration in this case.
rec3 employees%ROWTYPE;
-- Or we can mix fields that are table columns with user-defined fields.
TYPE rec4_t IS RECORD (first_name employees.first_name%TYPE, last_name employees.last_name%TYPE, rating NUMBER);
rec4 rec4_t;
BEGIN
-- Read and write fields using dot notation
rec1.field1 := 'Yesterday';
rec1.field2 := 65;
rec1.field3 := TRUNC(SYSDATE-1);
-- We didn't fill in the NAME field, so it takes the default value declared above.
dbms_output.put_line(rec2.name);
END;
/

To store a record in the database, you can specify it in an INSERT or UPDATE statement, if its fields match the columns in the table:

...

You can use %TYPE to specify a field type corresponding to a table column type. Your code keeps working even if the column type is changed (for example, to increase the length of a VARCHAR2 or the precision of a NUMBER). The following example defines RECORD types to hold information about a department:

DECLARE
-- Best: use %ROWTYPE instead of specifying each column.
-- Using <cursor>%ROWTYPE instead of <table>%ROWTYPE since we only want some columns.
-- Declaring the cursor doesn't run the query, so no performance hit.
CURSOR c1 IS SELECT department_id, department_name, location_id FROM departments;
rec1 c1%ROWTYPE;
-- Use <column>%TYPE in field declarations to avoid problems if the column types change.
TYPE DeptRec2 IS RECORD (dept_id departments.department_id%TYPE, dept_name departments.department_name%TYPE, dept_loc departments.location_id%TYPE);
rec2 DeptRec2;
-- Final technique, writing out each field name and specifying the type directly, is
-- clumsy and unmaintainable for working with table data. Use only for all-PL/SQL code.
TYPE DeptRec3 IS RECORD (dept_id NUMBER, dept_name VARCHAR2(14), dept_loc VARCHAR2(13));
rec3 DeptRec3;
BEGIN
NULL;
END;
/

PL/SQL lets you define records that contain objects, collections, and other records (called nested records). However, records cannot be attributes of object types.

Using Records as Procedure Parameters and Function Return Values

Records are easy to process using stored procedures because you can pass just one parameter, instead of a separate parameter for each field. For example, you might fetch a table row from the EMPLOYEES table into a record, then pass that row as a parameter to a function that computed that employee's vacation allowance or some other abstract value. The function could access all the information about that employee by referring to the fields in the record.

The next example shows how to return a record from a function. To make the record type visible across multiple stored functions and stored procedures, declare the record type in a package specification.

You can assign a %ROWTYPE record to a user-defined record if their fields match in number and order, and corresponding fields have the same datatypes:

DECLARE
TYPE RecordTyp IS RECORD (last employees.last_name%TYPE, id employees.employee_id%TYPE);
CURSOR c1 IS SELECT last_name, employee_id FROM employees;
-- Rec1 and rec2 have different types. But because rec2 is based on a %ROWTYPE, we can
-- assign is to rec1 as long as they have the right number of fields and the fields
-- have the right datatypes.
rec1 RecordTyp;
rec2 c1%ROWTYPE;
BEGIN
SELECT last_name, employee_id INTO rec2 FROM employees WHERE ROWNUM < 2;
rec1 := rec2;
dbms_output.put_line('Employee #' || rec1.id || ' = ' || rec1.last);
END;
/

You can also use the SELECT or FETCH statement to fetch column values into a record. The columns in the select-list must appear in the same order as the fields in your record.

You cannot assign a list of values to a record using an assignment statement. There is no constructor-like notation for records.

Comparing Records

Records cannot be tested for nullity, or compared for equality, or inequality.

If you want to make such comparisons, write your own function that accepts two records as parameters and does the appropriate checks or comparisons on the corresponding fields.

Inserting PL/SQL Records into the Database

A PL/SQL-only extension of the INSERT statement lets you insert records into database rows, using a single variable of type RECORD or %ROWTYPE in the VALUES clause instead of a list of fields. That makes your code more readable and maintainable.

If you issue the INSERT through the FORALL statement, you can insert values from an entire collection of records.

The number of fields in the record must equal the number of columns listed in the INTO clause, and corresponding fields and columns must have compatible datatypes. To make sure the record is compatible with the table, you might find it most convenient to declare the variable as the type table_name%ROWTYPE.

Example 5-34 Inserting a PL/SQL Record Using %ROWTYPE

This example declares a record variable using a %ROWTYPE qualifier. You can insert this variable without specifying a column list. The %ROWTYPE declaration ensures that the record attributes have exactly the same names and types as the table columns.

DECLARE
dept_info dept%ROWTYPE;
BEGIN
-- deptno, dname, and loc are the table columns.
-- The record picks up these names from the %ROWTYPE.
dept_info.deptno := 70;
dept_info.dname := 'PERSONNEL';
dept_info.loc := 'DALLAS';
-- Using the %ROWTYPE means we can leave out the column list
-- (deptno, dname, loc) from the INSERT statement.
INSERT INTO dept VALUES dept_info;
END;
/

Updating the Database with PL/SQL Record Values

A PL/SQL-only extension of the UPDATE statement lets you update database rows using a single variable of type RECORD or %ROWTYPE on the right side of the SET clause, instead of a list of fields.

If you issue the UPDATE through the FORALL statement, you can update a set of rows using values from an entire collection of records.

Also with an UPDATE statement, you can specify a record in the RETURNING clause to retrieve new values into a record. If you issue the UPDATE through the FORALL statement, you can retrieve new values from a set of updated rows into a collection of records.

The number of fields in the record must equal the number of columns listed in the SET clause, and corresponding fields and columns must have compatible datatypes.

The argument to SET ROW must be a real PL/SQL record, not a subquery that returns a single row.

The record can also contain collections or objects.

Example 5-36 Using the RETURNING Clause with a Record

The INSERT, UPDATE, and DELETE statements can include a RETURNING clause, which returns column values from the affected row into a PL/SQL record variable. This eliminates the need to SELECT the row after an insert or update, or before a delete.

By default, you can use this clause only when operating on exactly one row. When you use bulk SQL, you can use the form RETURNING BULK COLLECT INTO to store the results in one or more collections.

The following example updates the salary of an employee and retrieves the employee's name, job title, and new salary into a record variable:

Restrictions on Record Inserts/Updates

Currently, the following restrictions apply to record inserts/updates:

Record variables are allowed only in the following places:

On the right side of the SET clause in an UPDATE statement

In the VALUES clause of an INSERT statement

In the INTO subclause of a RETURNING clause

Record variables are not allowed in a SELECT list, WHERE clause, GROUPBY clause, or ORDERBY clause.

The keyword ROW is allowed only on the left side of a SET clause. Also, you cannot use ROW with a subquery.

In an UPDATE statement, only one SET clause is allowed if ROW is used.

If the VALUES clause of an INSERT statement contains a record variable, no other variable or value is allowed in the clause.

If the INTO subclause of a RETURNING clause contains a record variable, no other variable or value is allowed in the subclause.

The following are not supported:

Nested record types

Functions that return a record

Record inserts/updates using the EXECUTE IMMEDIATE statement.

Querying Data into Collections of Records

You can use the BULK COLLECT clause with a SELECT INTO or FETCH statement to retrieve a set of rows into a collection of records.

DECLARE
TYPE EmployeeSet IS TABLE OF employees%ROWTYPE;
underpaid EmployeeSet; -- Holds set of rows from EMPLOYEES table.
CURSOR c1 IS SELECT first_name, last_name FROM employees;
TYPE NameSet IS TABLE OF c1%ROWTYPE;
some_names NameSet; -- Holds set of partial rows from EMPLOYEES table.
BEGIN
-- With one query, we bring all the relevant data into the collection of records.
SELECT * BULK COLLECT INTO underpaid FROM employees
WHERE salary < 2500 ORDER BY salary DESC;
-- Now we can process the data by examining the collection, or passing it to
-- a separate procedure, instead of writing a loop to FETCH each row.
dbms_output.put_line(underpaid.COUNT || ' people make less than 2500.');
FOR i IN underpaid.FIRST .. underpaid.LAST
LOOP
dbms_output.put_line(underpaid(i).last_name || ' makes ' || underpaid(i).salary);
END LOOP;
-- We can also bring in just some of the table columns.
-- Here we get the first and last names of 10 arbitrary employees.
SELECT first_name, last_name BULK COLLECT INTO some_names FROM employees
WHERE ROWNUM < 11;
FOR i IN some_names.FIRST .. some_names.LAST
LOOP
dbms_output.put_line('Employee = ' || some_names(i).first_name || ' ' || some_names(i).last_name);
END LOOP;
END;
/